In general, heaters including electric resistance heating elements are well known in the art. Such heaters rely upon external electrical control mechanisms to adjust the temperature of such resistance heating elements. To attain a desired temperature, such heating elements are cycled on and off to maintain the heating elements within a prescribed range of temperatures. Such heating elements fail to provide uniform heating throughout the resistance elements. That is, such heating elements generally exhibit hot spots and thus do not provide uniform heating at a desired temperature throughout the entire volume of the heating element.
In the metallurgical field, induction heaters are commonly used to melt metal. In particular, a crucible containing a metal charge to be melted is placed within an induction coil, and an alternating current is passed through the induction coil to cause the metal charge to be melted.
The use of ferrite particles to produce heating in alternating magnetic fields is known in the art. As disclosed in U.S. Pat. No. 3,391,845 to White, and U.S. Pat. No. 3,902,940 to Heller et al., ferrite particles and other particles have been used to produce heat where it is desired to cause chemical reactions, melt materials or evaporate solvents.
U.S. Pat. No. 4,914,267 to Derbyshire (hereinafter "Derbyshire") relates to connectors containing fusible materials to assist in forming a connection, the connectors forming part of a circuit during the heating of the fusible material. In particular, the temperature of the connectors is auto-regulated at about the Curie temperature of the magnetic material included in the circuit during the heating operations. The connector may be a ferromagnetic member or may be a part of a circuit including a separate ferromagnetic member.
Derbyshire explains that auto-regulation occurs as a result of the change in value of mu (a measure of the ferromagnetic properties of the ferromagnetic member) to approximately 1 when the Curie temperature is approached. In particular, the current spreads into the body of the connector thus lowering the concentration of current in a thin layer of magnetic material, and the skin depth changes by at least the change in the square root of mu. Resistance to current flow reduces, and if the current is held at a constant value, the heating effect is reduced below the Curie temperature, and the cycle repeats. Thus, the system auto-regulates about the Curie temperature.
Derbyshire discloses embodiments wherein the connector is made of ferromagnetic material, a high frequency constant current a.c. is passed through the ferromagnetic material causing the connector to heat until its Curie temperature is reached. When this happens, the effective resistance of the connector reduces and the power dissipation falls such that by proper selection of current, frequency and resistivity and thickness of materials, the temperature is maintained at about the Curie temperature of the magnetic material of the connector. In another embodiment, a laminar ferromagnetic-non-magnetic heater construction comprises a copper wire, tube, rod or other metallic element in a ferromagnetic sleeve. In this case, current at proper frequency applied to opposite ends of the sleeves flows through the sleeve due to the skin effect until the Curie temperature is reached, at which time the current flows primarily through the copper wire. In a still further embodiment, the connector includes a copper sleeve with axially-spaced rings of high mu materials of different Curie temperatures so as to produce different temperatures displaced in time and space.
An object of this invention is to provide a heater device having improved properties and utility.